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Physics Focus Dice Become Ordered When Stirred Not Shaken .pdf

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Physics - Focus: Dice Become Ordered When Stirred, Not Shaken

Focus: Dice Become Ordered When Stirred, Not Shaken
December 1, 2017 • Physics 10, 130
A jumble of thousands of cubic dice, agitated by an oscillating rotation, can rapidly become completely ordered, a result that is hard to
produce with more conventional shaking.

K. Asencio et al., Phys. Rev. Lett. (2017)

A twist of the dice. Su iciently vigorous twisting of a cylinder containing 25,000 randomly-oriented dice
(le ) leads to a nearly perfectly ordered arrangement (right). The more conventional technique of
tapping the system takes a long time and requires a carefully-controlled “annealing” procedure. (See
videos below.) Show less

Repeatedly tap on a box of marbles or sand and the pieces will pack themselves more tightly with each tap. However, the contents will
only approach its maximum density a er a long time and if you use a carefully cra ed tapping sequence. But in new experiments with a
cylinder full of dice vigorously twisted back and forth, the pieces achieved their maximum density quickly. The experiments could point
to new methods to produce dense and technologically useful granular systems, even in the zero gravity environments of space
Granular matter is common in nature and in industry, from the sand and gravel of the building trade to powders in foods and
pharmaceutical manufacturing. Under the influence of gravity, simple vibration or tapping can make granular matter grow more
compact over time, as dislodged particles find ways to fall further down with each shake. Industrial plants use this e ect with powders
and grains of many kinds.




Physics - Focus: Dice Become Ordered When Stirred, Not Shaken

K. Asencio et al., Phys. Rev. Lett. (2017)

With the angular acceleration below the threshold, the twisting oscillation does not lead to complete
ordering of the dice in the center, even a er a day of agitation (100,000 twists), as shown in this timelapse video. (A logarithmic time scale is used, with identical intervals of time passing between images for
100; 1000; 10,000; and 100,000 cycles.)

Exploring other ways to pack granular matter, Diego Maza and colleagues from the University of Navarra in Spain experimented with
plastic dice in an upright cylinder subjected to oscillating rotations about its vertical axis. They observed gradual packing but also found
that the e ects of twisting are quite di erent from those of tapping.
The team first poured 25,000 dice, each roughly a half centimeter on a side, into a cylindrical container. They then began twisting the
container, rotating it alternately clockwise and counterclockwise about once per second and continued for several hundred thousand
twists. The rotation exerted a force on the dice directed outward toward the walls, and the oscillation provided periodic jolts at the two
moments during each cycle when the rotation direction reversed. The team varied the strength of these jolts by adjusting the speed of
the steady rotation between reversals.

K. Asencio et al., Phys. Rev. Lett. (2017)

With the angular acceleration above the threshold, the twisting oscillation leads to almost completely
ordered dice a er about 10,000 twists, as shown in this time-lapse video, which covers 100,000 twists. (A
logarithmic time scale is used, with identical intervals of time passing between images for 100; 1000;
10,000; and 100,000 cycles.)

The results show that the degree of dice ordering depends strongly on the rotational acceleration applied at each reversal. For
accelerations above about half the acceleration of gravity (0.5 g), the dice reach a final state of maximum density a er about 10,000
twists. In this state, the dice lie in horizontal layers within the cylinder and also in nearly perfectly ordered concentric rings within each
layer. In contrast, for accelerations below 0.5 g, the packing proceeds so slowly that it is unclear if the dice would ever reach the state of
maximum density. Even a er 100,000 twists, the dice near the center of the cylinder remain highly disorganized. Maza and colleagues
estimate that it might take ten years of twisting to reach the maximum density.
These results, says Maza, show a very di erent packing process for twisting than for tapping. A tapped granular system, even a
collection of cubes, won't reach a state of maximum density on its own but tends to get stuck in a state of intermediate density. It can
only be made more dense through a process called annealing, in which the tapping grows more gentle over time. In contrast, for strong
enough twisting, the dice always reach the most dense state without any annealing.
“The experiments are solid and quite clean,” says statistical physicist Matthias Schröter of the Max Planck Institute for Dynamics and
Self-Organization in Göttingen, Germany, who suggests that the process by which the dice become ordered is similar to that of other
systems. For example, contact with a boundary can trigger the formation of crystalline order in a collection of colloidal particles
suspended in a liquid; the dice may be similarly influenced by the outer cylinder wall. “What they have observed,” says Schröter, “is
heterogeneous nucleation, or the growth of crystalline order induced by the boundaries.”




Physics - Focus: Dice Become Ordered When Stirred, Not Shaken

Maza agrees that this might well be the explanation and hopes the method might find uses in settings where the usual packing by
tapping procedure doesn’t work, such as in the absence of gravity. Maza and his colleagues are preparing experiments for the
International Space Station, aiming to study the dynamics of granular materials under microgravity conditions. To achieve useful
densities, they've been using moving pistons to produce vibrations roughly equivalent to tapping, but they may explore the twisting
method in future work.
This research is published in Physical Review Letters.
–Mark Buchanan
Mark Buchanan is a freelance science writer who splits his time between Wales, UK, and Normandy, France.

Subject Areas
So Matter

Experimental Study of Ordering of Hard Cubes by Shearing
K. Asencio, M. Acevedo, I. Zuriguel, and D. Maza
Phys. Rev. Lett. 119, 228002 (2017)
Published December 1, 2017



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